It is known that firstly a hemiaminal is formed from an aldehyde and a secondary amine (amination) and this can also further react with another molecule of the amine to give the aminal with the elimination of water.
All of the specified reaction products (hemiaminal and aminal) can be hydrogenated catalytically to the corresponding amines. This reduction is described for example in Houben-Weyl, Methoden der Organischen Chemie [Methods of organic chemistry], 4th edition, volume IV/1c (1980), p. 127/128, 239/240 and 438. Such catalytic hydrogenations are also supposed to be possible while retaining halogen. However, this result is limited to the use of palladium catalysts, and it is further indicated that it is advantageous to work with deactivated catalyst and at relatively low temperatures (Houben-Weyl, Methoden der Organischen Chemie [Methods of organic chemistry], 4th edition, volume IV/1c (1980), page 240). Halogen is also supposed to be retained when using other catalysts such as platinum or Raney nickel. However, the specific hydrogenations indicated in the cited passages do not constitute a systematic investigation (Houben-Weyl, Methoden der Organischen Chemie [Methods of organic chemistry], 4th edition, volume IV/1c (1980), page 436, paragraph 3) and at times exhibit extremely moderate yields, as in the case of p-chlorobenzyl methyl ketone, which can be converted to the appertaining amine only in 10% of the theoretical yield (Houben-Weyl, Methoden der Organischen Chemie [Methods of organic chemistry], 4th edition, volume IV/1c (1980), page 436, at the bottom), Particularly when producing strongly basic amines, by-products have to be expected (Houben-Weyl, Methoden der Organischen Chemie [Methods of organic chemistry], 4th edition, volume IV/1c (1980), page 240, paragraph 2).
EP-A-0 355 351 discloses the catalytic hydrogenation in the presence of sulfur in the form of organic sulfur compounds. However, sulfur compounds have the disadvantage that they have to be precisely metered in since they poison the catalyst in the event of an incorrect dose or overdose. Moreover, sulfur compounds have negative effects on the end product, particularly chloro-N,N-dimethylbenzylamine, meaning that they have to be separated off, which entails expense.
Furthermore, WO 20091110985 discloses producing halogen-N,N-dimethylbenzylamines wherein halogen bromine from sodium cyanoborohydride. However, this method has the disadvantages that cyanides are released since sodium cyanoborohydride has to be used in a high excess and, as a consequence of this, a high salt content is produced which has to be disposed of in a costly manner. These methods are not suitable on an industrial scale on account of the high salt content and the toxicity of the sodium cyanoborohydride.
Alternative methods, as described in EPA 0355315, provide the addition of solid co-catalysts which have the disadvantage that they can bleed, meaning that they are found as undesired by-product in the end product.
In the methods according to the prior art, either expensive precious metal catalysts are used, only low yields are achieved, or the added additives have to be separated off in an expensive manner, which often renders these methods uneconomical.
Against the background of the aforementioned prior art, the object was therefore to provide a particularly economical method which permits the production of halogen-N,N-dimethylbenzylamines wherein halogen Cl or Br, preferably chloro-N,N-dimethylbenzylamine, particularly preferably ortho-chloro-N,N-dimethylbenzylamine (o-Cl-DMBA), in high yields and in the absence of sulfur and sulfur-containing compounds or other catalyst poisons or additives which reduce the catalyst activity.